1. Technical Field of the Invention
The present invention relates to a cooled stationary blade for a gas turbine, and more particularly to a cooled stationary blade for a cooled blade type gas turbine using both steam cooling and air cooling.
2. Description of the Related Art
Currently, the cooling of stationary blades of a gas turbine is performed by air, which consumes a large amount of cooling air. Consequently, a large amount of cooling air is fed to the blades and a large amount of cooling air leaks out resulting in a degradation of performance of the gas turbine. A typical conventional air cooling system for the gas turbine stationary blades will now be described.
FIG. 5 shows an example of inserts in a gas turbine stationary blade. FIG. 6 is a cross-sectional view thereof. In both drawings, reference numeral 30 denotes the overall stationary blade, numeral 31 denotes an outer shroud and numeral 32 denotes an inner shroud. Numerals 33, 34 and 35 denote inserts in the stationary blade whose interior is divided into three sections. Numeral 36 denotes a rear edge fin. The stationary blade is composed of the outer shroud, the inner shroud, the insert and the rear edge fin.
High pressure air 38 is fed from the outer shroud 31 into each insert 33, 34 and 35. The high pressure cooling air is injected toward the blade from a number of cooling air holes 33a, 34a and 35a formed in each insert 33, 34 and 35 to thereby impingement cool the inner surface of the blade. Afterwards, the cooling air is injected to the outside of the blade from cooling air holes 37a, 37b, 37c and 37d provided in the blade surface. Shower head cooling, film cooling and pin fin cooling are performed at the front edge portion of the blade, the back and front portions of the blade and the rear edge fin 36 portion, respectively.
FIG. 7 shows a cooling system in another conventional gas turbine stationary blade. FIG. 8 is a cross-sectional view thereof. Numeral 40 denotes the overall stationary blade, numeral 41 denotes an outer shroud and numeral 42 denotes an inner shroud. In the interior of the blade, air passages 43A, 43B, 43C, 43D and 43E are communicated with each other at upper and lower portions thereof to form a serpentine cooling path. Numeral 45 denotes a fin provided at a rear edge. A number of air cooling holes 44 for blowing the cooling air out from the passage 43E are provided. Numeral 46 denotes a number of turbulators provided in the interior of each of the air passages 43A to 43E for enhancing heat transmission.
The cooling air 47 is fed from the upper portion of the air passage 43A located at the front edge of the outer shroud 41 and flows to the lower portion thereof, to enter the air passage 43B, flows upwardly in the passage 43B, enters the passage 43C from the upper portion of the air passage 43B, and flows in the passages 43D and 43E in a similar manner to cool the blade by means of the respective passages. The air after cooling flows out of the blade from the cooling air holes 44 of the rear edge fin 45 and the rest of the air flows out of the blade from the lower portion of the air passage 43E.
As is apparent from the above-described examples shown in FIGS. 5 to 8, a large amount of cooling air is required to cool the stationary blade in the conventional gas turbine. Currently, a large amount of power is also required for a compressor or a cooler.
As described above, in the conventional gas turbine stationary blade, a large amount of cooling air always flows through the blade for cooling, and an amount of air is required to seal the blade in order to prevent the high temperature combustion gas from being introduced into the interior of the blade from the gas passage. Consequently, a relatively large amount of power is consumed by the compressor or the cooler in order to keep the air at a high pressure. Thus, the conventional system suffers from a problem in that the performance of the gas turbine is degraded.
Also, recently, a combined cycle for enhancing power generating efficiency by combining a gas turbine and a steam turbine has been realized. A variety of publications disclose that, instead of using air to cool the blade, a portion of the steam generated in the steam turbine is extracted and this steam is introduced into the blade. However, by only simply applying the air cooling model there are problems which have yet to be solved such as: how the steam is introduced into the stationary blade or the shroud, which portion of the stationary blade and the shroud are effectively cooled, what structure the cooling passage should have and how the steam should be collected.